Loudspeakers and microphones essentially comprise a movable diaphragm or other member which provides conversion between a sound pressure wave and an electrical signal.
Microphones are moving from typical analogue microphones to digital microphone modules. These microphone modules typically consist of a sensor manufactured in a micro-electro-mechanical system (MEMS) process and an analogue to digital converter (ADC). The output of the ADC (typically a sigma delta type converter) is a PDM (pulse density modulation) stream that outputs the data to a baseband processor.
MEMS microphones offer a cheaper and more reproducible alternative to traditional electret condenser microphones (ECM) for mass market applications such as GSM.
The membrane in a MEMS microphone is typically a silicon membrane, which is suspended over a base substrate. The base substrate is glued to a laminate, which carries other components, such as ICs for processing the microphone signals. The response of a MEMS microphone is greatly influenced by the stress in the Si membrane that moves with changes in sound pressure. This stress can be affected greatly by strain in the substrate induced by differences in thermal expansion coefficient, for example between the base substrate of the MEMS microphone structure and a laminate on which it is glued.